Low ambient cooling scheme and control

ABSTRACT

A system and method for controlling pressure during low ambient conditions when operating in a cooling mode are provided. One or more controllers may be operable to receive a measurement of a first head pressure at a refrigerant line and determine whether the first head pressure is below a pre-determined minimum head pressure threshold. The one or more controllers may be further operable to send an instruction to open a hot gas bypass valve in response to a determination that the first head pressure is below the pre-determined minimum head pressure threshold and a speed of one or more outdoor fan motors is less than or equal to a pre-determined minimum operating fan speed.

TECHNICAL FIELD

This application is directed, in general, to heating, ventilation, and air conditioning systems (HVAC) and, more specifically, to low ambient cooling scheme and control.

BACKGROUND

A variable refrigerant flow (“VRF”) system is an HVAC system that typically utilizes an outdoor condensing unit and multiple indoor fan-coil units. A VRF system can be adjusted to provide for individual heating and cooling needs for different spaces within a building. The system responds to indoor loading variation by adjusting the outdoor compressor speed and controls, which allows refrigerant to be delivered to the individual indoor fan coil units at variable rates depending on the individual heating and/or cooling needs of each space. Whereas in conventional HVAC systems the compressor cycles on and off, in VRF systems the compressor operates continuously at varying speeds under normal ambient conditions.

SUMMARY

According to one embodiment, a system for controlling head pressure during low ambient conditions comprises a condenser, an outdoor expansion valve, an indoor expansion valve, a hot gas bypass valve, one or more outdoor fans, one or more outdoor fan motors, and one or more controllers. The condenser is operable to receive refrigerant from a discharge line, condense the refrigerant, and discharge the refrigerant to a first portion of a refrigerant line. The outdoor expansion valve is disposed between the first portion of the refrigerant line and a second portion of the refrigerant line, and the indoor expansion valve is disposed between the second portion of the refrigerant line and a third portion of the refrigerant line. The hot gas bypass valve is coupled to the discharge line and to the second portion of the refrigerant line such that, when open, refrigerant through the hot gas bypass valve bypasses the condenser and the outdoor expansion valve. The one or more outdoor fans are operable to cool the condenser, and the one or more outdoor fan motors are operable to control the fan speed of the one or more outdoor fans. The one or more outdoor fan motors have a pre-determined minimum operating fan speed. The one or more controllers are operable to receive a measurement of a first head pressure at the refrigerant line (e.g., at the first portion and/or second portion of the refrigerant line) and determine whether the first head pressure is below a pre-determined minimum head pressure threshold. The one or more processors are further operable to send an instruction to at least partially open the hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold, and a speed of the one or more outdoor fan motors is less than or equal to the pre-determined minimum operating fan speed.

According to another embodiment, a method for controlling head pressure in an HVAC system during low ambient conditions comprises receiving, by a processor, a measurement of a first head pressure at a refrigerant line and determining, by the processor, whether the first head pressure is below a pre-determined minimum head pressure threshold. The method further comprises sending, by the processor, an instruction to at least partially open a hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold, and a speed of one or more outdoor fans is less than or equal to a pre-determined minimum operating speed of the one or more outdoor fans.

Advantageously, aspects of the present disclosure may allow for the HVAC system to operate in low ambient conditions while maintaining smooth operation. Another advantageous aspect of the present disclosure may provide a method of smoothly operating a VRF or HVAC system beyond the lowest speed of outdoor fans. Further, aspects of the present disclosure may maintain the HVAC system operation at optimal conditions while the HVAC system operates in low ambient conditions. Aspects of the present disclosure may be utilized, therefore, to prevent unstable suction pressure and capacity output, thus providing smooth operation of the HVAC system during low ambient conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an example HVAC system for controlling head pressure during low ambient conditions in accordance with embodiments of the present disclosure;

FIG. 2 illustrates an example controller that may be utilized in an HVAC system for controlling head pressure during low ambient conditions in accordance with embodiments of the present disclosure;

FIG. 3 illustrates an example HVAC system for controlling head pressure during low ambient conditions in accordance with embodiments of the present disclosure; and

FIG. 4 illustrates an example method for controlling head pressure in an HVAC system during low ambient conditions in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

In general, VRF systems may provide smooth operation and close response to indoor loading variation by adjusting the compressor speed and controls. In certain situations, it may be desirable to operate a VRF system in a cooling mode during low ambient conditions. For example, in the winter, the outdoor ambient temperature may be below freezing. However, it may be desirable to operate the VRF system in a cooling mode during such low ambient conditions, for example, if the indoor space conditioned by the VRF system houses a number of computers that generate heat within the conditioned space.

When in cooling mode, refrigerant may pass through an outdoor condenser that cools the refrigerant. Cooling the refrigerant generally causes the pressure of the refrigerant to decrease. In normal ambient conditions, the amount of cooling and thus the amount of pressure decrease can be controlled by adjusting the speed of one or more outdoor fans that circulate ambient air across the outdoor condenser. The fan speed may be increased for more cooling and decreased for less cooling. In low ambient conditions, one or more of the outdoor fans of a VRF system slows down to reduce the heat rejection and maintain proper head pressure. However, the fan motors used to drive the outdoor fans have a minimum speed to ensure reliable operation, and when the ambient is very cold, the head pressure cannot be maintained even when the fan speed is at a minimum. Traditionally, the fan will be cycled on and off based on predetermined liquid pressures when the lowest fan speed fails to maintain the head pressure. This may cause unstable suction pressure and capacity output.

Certain embodiments of the present disclosure may allow for smoothly operating a VRF system beyond the lowest speed of the outdoor fan. For example, in certain embodiments, if the head pressure is below a pre-determined threshold and the fan speed is at the lowest fan speed, a hot gas bypass valve may be at least partially opened so that at least part of the refrigerant bypasses the condenser. The refrigerant that bypasses the condenser maintains a high temperature and high pressure. The refrigerant that bypasses the condenser may be combined with cooler, lower pressure refrigerant that has passed through the condenser to obtain an appropriate temperature and pressure. Examples of systems and methods for low ambient cooling control are further described with respect to FIGS. 1-4 below.

Referring to FIG. 1, an example embodiment of an HVAC system 100 for controlling head pressure during low ambient conditions is shown. HVAC system 100 may be any type of system for controlling head pressure during low ambient conditions. For example, HVAC system 100 may be a VRF system that responds to indoor loading variation by controlling the amount of refrigerant provided to indoor units. HVAC system 100 may be a heat recovery system, a heat pump system, a split or packaged system, or any other system suitable for use in HVAC applications.

HVAC system 100 may be provided with a component configuration capable of cooling only, heating only, or both heating and cooling operation. For example, referring to the embodiment of FIG. 1, HVAC system 100 components may be configured for a cooling operation with the refrigerant of HVAC system 100 flowing in the directions indicated by the arrows of FIG. 1. HVAC system 100 may be used in any suitable environment, such as commercial, industrial, and residential buildings, and in refrigeration. The capacity of HVAC system 100 may be any capacity suitable for its intended purpose. For example, the capacity of HVAC system 100 may range from 6 tons to 36 tons for commercial or industrial applications and the capacity of HVAC system 100 may range from 3 tons to 5 tons for residential applications.

Referring to FIG. 1, HVAC system 100 may include an outdoor unit 110, a controller 112, a compressor 120, a discharge line 126, a condenser 130, a fan 132, a fan motor 134, an outdoor expansion valve 140, a hot gas bypass valve 150 (e.g., a solenoid valve), a refrigerant line 155, an indoor expansion valve 160, an evaporator 170, and a pressure sensor 180. In the illustrated embodiment, outdoor unit 110, compressor 120, condenser 130, fan 132, fan motor 134, outdoor expansion valve 140, hot gas bypass valve 150, and pressure sensor 180 are located outside, whereas indoor expansion valve 160 and evaporator 170 are located inside. In other embodiments, one or more of the HVAC components shown outside may be located or partially located inside. Similarly, one or more of the HVAC components shown inside may be located or partially located outside. Further, HVAC system 100 may include additional, fewer, or different components than those shown in the embodiment of FIG. 1. For example, HVAC system 100 may include additional, or fewer, sensors 180, which may be disposed at a location, or locations, differing from that shown.

Outdoor unit 110 of HVAC system 100, as illustrated in FIG. 1, may comprise an enclosure, compressor 120, condenser 130, fan 132, fan motor 134, outdoor expansion valve 140, hot gas bypass valve 150, and pressure sensor 180. Outdoor unit 110 may be disposed at an outdoor location, such as on a slab adjacent to a building or on the roof of a building, for example. In alternative embodiments, outdoor unit 110 may include additional, fewer, or different components than those shown in the embodiment of FIG. 1. For example, outdoor unit 110 may include additional outdoor fans 132 along with corresponding additional motors 134.

As shown in FIG. 1, HVAC system 100 may comprise controller 112. Controller 112 may be implemented with logic for receiving measurements, making determinations, and sending instructions. In certain embodiments, controller 112 comprises a memory, a processor, and an interface analogous to those described below with respect to FIG. 2.

In an embodiment, controller 112 may receive a measurement of a head pressure and determine whether the head pressure is below a pre-determined minimum head pressure threshold. For example, controller 112 may determine that the received head pressure measurement is below a pre-determined minimum head pressure threshold. Any suitable pre-determined minimum head pressure threshold may be used depending on the system. In certain embodiments, the pre-determined minimum head pressure threshold may between 200 and 300 pounds per square inch gauge (“psig”), such as 250 psig, for example, when using R-410A as the refrigerant for certain embodiments of the HVAC system. Additionally, or alternatively, controller 112 may receive a measurement of a head pressure and determine whether the head pressure exceeds a pre-determined maximum head pressure threshold. Any suitable pre-determined maximum head pressure threshold may be used depending on the system. In certain embodiments, the pre-determined maximum head pressure threshold may between 400 and 500 psig, such as 450 psig.

Controller 112 may selectively energize, de-energize, or configure HVAC system 100 components. For example, in an embodiment, controller 112 may set the operating speed of a compressor, motor, or the like. In some embodiments, controller 112 may be operable to send an instruction to partially or fully open or close one or more valves of HVAC system 100. Additionally, or alternatively, controller 112 may be operable to increase or decrease the speed of a fan motor of HVAC system 100. For instance, controller 112 may send an instruction to increase the speed of fan motor 134 of HVAC system 100 in response to a determination that a head pressure at a refrigerant line exceeds a pre-determined maximum head pressure threshold.

Controller 112 may operably couple to HVAC system 100 components via wired or wireless connections. For instance, as shown in FIG. 1, controller 112 is coupled to outdoor unit 110. In some embodiments, controller 112 may be located within an enclosure of outdoor unit 110. HVAC system 100 may include more than one controller. For example, outdoor unit 110 may be connected to or controlled by one or more controllers and the indoor components, such as indoor expansion valve 160 and evaporator 170, may be connected to or controlled by one or more controllers located indoors. Although certain examples have been described, any suitable number of controller(s) may be provided in any suitable location(s) to control the outdoor and/or indoor components.

As shown in the embodiment of FIG. 1, HVAC system 100 may include compressor 120 for compressing refrigerant as part of a vapor compression cycle. According to the embodiment shown in FIG. 1, compressor 120 may receive refrigerant via a suction line and may discharge compressed refrigerant into a discharge line 126. Discharge line 126 may couple to a discharge port of compressor 120. Discharge line 126 may direct the compressed refrigerant to condenser 130 of HVAC system 100.

Compressor 120 may be of any suitable type, such as a rotary compressor, a scroll compressor, a reciprocating compressor, or any other type of compressor suitable for use in HVAC system applications. Compressor 120 may be configured for variable speed or single speed operation. Compressor 120 may operably couple to controller 112 via a wired or wireless connection and may be selectively energized, de-energized, or set to a desired operating speed by controller 112 to meet a demand on HVAC system 100.

As shown, a single compressor 120 may be included in HVAC system 100. For instance, HVAC system 100 may include a single variable speed compressor. In alternative embodiments, HVAC system 100 may include more than one compressor 120. For example, HVAC system 100 may include a variable speed compressor plus a fixed-speed compressor. As another example, HVAC system may include multiple variable speed compressors. In such embodiments, the compressors provided may be configured to operate as a tandem compressor group. The tandem compressors may each be incorporated within a single circuit of HVAC components configured for vapor compression cycle operation, with each tandem compressor operatively coupling to the discharge line 126 and the suction line.

HVAC system 100, as shown in FIG. 1, may further comprise condenser 130. Condenser 130 may provide for heat transfer between the refrigerant of HVAC system 100 and airflow passing over coils of condenser 130 as part of the vapor compression cycle. Airflow passing over condenser 130 may comprise outdoor ambient air. Condenser 130 may be a coil-type heat exchanger of any known type commonly used in HVAC systems, such as a fin-and-tube heat exchanger coil, a microchannel heat exchanger coil, or the like. In some instances, condenser coil may comprise copper tubing with aluminum fins. In some specific instances, condenser coil may comprise rifled copper tubing with hydrophilic aluminum fins. Alternatively, condenser coil may comprise all-aluminum tubing.

According to the embodiment shown in FIG. 1, HVAC system 100 may be configured for cooling operation, with condenser 130 receiving refrigerant via discharge pipe 126. In this configuration, condenser 130 may be a condenser within a circuit of vapor compression cycle components. The refrigerant received by condenser 130 may be at a relatively high pressure and temperature gas phase refrigerant. The received refrigerant may reject heat and condense while flowing within condenser 130. High pressure liquid refrigerant may exit condenser 130 and flow into a high pressure liquid pipe. In alternative embodiments, HVAC system 100 may be configured to operate in heating mode. In such embodiments, condenser 130 within the outdoor unit 110 may be configured to operate as an evaporator as part of the vapor compression cycle.

HVAC system 100 may include outdoor fan 132 for inducing flow of ambient air over condenser 130. According to the embodiment shown, outdoor fan 132 may comprise a plurality of blades which may couple to, and be rotated about, a hub or shaft through actuation of fan motor 134. Energizing of fan motor 134 may cause rotation of outdoor fan 132 blades about the hub. Rotation of outdoor fan 132 blades may cause ambient air movement to induce a flow of ambient air over condenser 130. The ambient air flowing over condenser 130 may be at an ambient temperature.

Fan motor 134 may be an electric motor which may rotate in response to a received signal, or signals. Fan motor 134 may be configured to operate as a variable speed motor, whereby fan motor 134 may operate at a plurality of speeds, as measured in revolutions per minute (RPM). The speed of fan motor 134 may vary in response to changes to the signal, or signals, received. In such embodiments, fan motor 134 may be provided with a range of speed values within which fan motor 134 may be operated. The range may have a pre-determined minimum operating speed below which fan motor 134 may not operate. If commanded to operate at a speed outside of the operating range of fan motor 134, fan motor 134 may de-energize or may, alternatively, operate at a lowest speed setting. Similarly, the range of speed values within which fan motor 134 may be operated may have a pre-determined maximum operating speed.

In an embodiment, fan motor 134 may be an electric commutation (EC) type motor. The electrical input to fan motor 134 may be a direct current (DC) input or an alternating current (AC) input. The speed of fan motor 134 may be controlled using any known method of motor speed control. Fan motor 134 may operably connect to controller 112. Controller 112 may transmit control and/or power signals to fan motor 134 for varying the speed of fan motor 134. In an embodiment, in an embodiment, a controller may vary the control and/or power signal voltage to adjust the speed of fan motor 134. For example, controller 112 of HVAC system 100 may send an instruction to increase the speed of outdoor fan 134 if a head pressure at refrigerant line 155 exceeds a pre-determined maximum head pressure threshold. Alternatively, the controller may vary the pulse widths of a power and/or control signal transmitted to fan motor 134 to vary the speed of fan motor 134. During low ambient conditions, controller 112 may de-energize fan motor 134. Low ambient temperatures may comprise outdoor temperatures ranging from 0 degrees Fahrenheit to 23 degrees Fahrenheit. The controller may accordingly set the speed of fan motor 134 in response to conditions within HVAC system 100 in accordance with control logic executed by the controller.

HVAC system 100 may comprise outdoor expansion valve 140, as shown in FIG. 1. Outdoor expansion valve 140 may be any suitable type of expansion valve, such as electronic expansion valves (EXVs). As shown in the illustrated embodiment, outdoor expansion valve 140 may be disposed between and coupled to condenser 130 and indoor expansion valve 160. In some embodiments, outdoor expansion valve 140 may be located within an enclosure of outdoor unit 110. In the embodiment shown in FIG. 1, a single outdoor expansion valve 140 is provided. In alternative embodiments of HVAC system 100, additional outdoor expansion valves may be provided. For example, additional expansion valves may be provided in an HVAC system configured to operate as a dual flow, heat pump, VRF, and/or other HVAC system type. In other embodiments, the outdoor expansion valve of HVAC system 100 may be eliminated.

Refrigerant flow within HVAC system 100 may be directed through outdoor expansion valve 140 to indoor expansion valve 160. Outdoor expansion valve 140 may receive refrigerant from condenser 130 and deliver refrigerant to indoor expansion valve 160 via refrigerant line 155. Outdoor expansion valve 140 may be operably coupled to a controller via a wired or wireless connection. In an embodiment, a controller may be operable to send an instruction to at least partially close an outdoor expansion valve in response to a determination that a head pressure is below a pre-determined head pressure threshold. For example, controller 112 of HVAC system 100 may send an instruction to fully close outdoor expansion valve 140 in response to a determination that a head pressure is below a pre-determined head pressure threshold of 200 psig.

In the embodiment shown in FIG. 1, HVAC system 100 comprises hot gas bypass valve 150. Hot gas bypass valve 150 may be disposed between discharge line 126 and refrigerant line 155. In the illustrated embodiment, discharge line 126 is located between compressor 120 and condenser 130, and refrigerant line 155 is located between outdoor expansion valve 140 and indoor expansion valve 160. Hot gas bypass valve 150 may be operable to introduce a portion of high pressure, high temperature gas from discharge line 126 to refrigerant line 155. In an embodiment, hot gas bypass valve 150 may be used to manipulate the head pressure within HVAC system 100 in response to changing ambient conditions as well as changing conditions within HVAC system 100 during operation. For example, in low ambient conditions when the head pressure in refrigerant line 155 is below a pre-determined threshold, hot gas bypass valve 150 of system 100 may be opened to introduce hot gas from compressor 120 into refrigerant line 155. The hot gas may mix with the refrigerant in refrigerant line 155 to increase the head pressure in refrigerant line 155.

In an embodiment, hot gas bypass valve 150 may be operably coupled to a controller via a wired or wireless connection. The controller may be operable to at least partially open hot gas bypass valve 150 in response to certain determinations. For example, controller 112 may fully open hot gas bypass valve 150 when a received head pressure measurement is below a pre-determined head pressure threshold and outdoor fan motor 134 is operating at a speed less than or equal to a pre-determined operating fan speed. Similarly, in some embodiments, controller 112 may be operable to at least partially close hot gas bypass valve 150 in response to certain determinations.

Hot gas bypass valve 150 and outdoor expansion valve 140 may work in conjunction to maintain the head pressure in refrigerant line 155 within a pre-determined head pressure range. In an embodiment, controller 112 may partially or fully open or close hot gas bypass valve 150 and/or outdoor expansion valve 140 to maintain a head pressure within 250 psig and 450 psig at refrigerant line 155. In certain embodiments, a minimum head pressure threshold of HVAC system 100 may be a value between 200 and 300 psig, whereas a maximum head pressure threshold may be a value between 400 psig and 500 psig. However, those of ordinary skill in the art will appreciate that the pre-determined head pressure range of system 100 will vary depending on conditions such as the type of refrigerant used in the HVAC system and the HVAC system's configuration. In some embodiments, the minimum head pressure threshold may be based on the pressure drop required by indoor expansion valve 160 to meet the demands of evaporator 170's load.

As shown in FIG. 1, in an embodiment, HVAC system 100 may include indoor expansion valve 160. Indoor expansion valve 160 may be any suitable type of expansion valve, including an electronic expansion valve (EXVs), a thermal expansion valve (TXVs), a short orifice, and the like. Refrigerant flow within HVAC system 100 may be directed through indoor expansion valve 160 to evaporator 170. Indoor expansion valve 160 may remove pressure from the liquid refrigerant to allow expansion or change of state from a liquid to a vapor in the evaporator. For example, indoor expansion valve 160 may couple with, and receive high temperature and pressure liquid refrigerant from, a high pressure liquid pipe. Indoor expansion valve 160 may also couple with, and deliver low pressure refrigerant to, a low pressure pipe. In an embodiment, indoor expansion valve 160 may be operably coupled to a controller via a wired or wireless connection.

As shown in the illustrated embodiment, indoor expansion valve 160 may be disposed between outdoor expansion valve 140 and evaporator 170 of HVAC system 100, as part of a vapor compression cycle. According to HVAC system 100 embodiment shown, a single indoor expansion valve 160 is provided. In alternative embodiments of HVAC system 100, additional indoor expansion valves may be provided. For example, additional indoor expansion valves may be provided in an HVAC system configured to operate as a dual flow, heat pump, VRF, and/or other HVAC system type. Additionally, some embodiments may include multiple indoor expansion valves and no outdoor expansion valves. The operation of such indoor expansion valves is well known to those of ordinary skill in the art and is omitted from this description.

In alternative embodiments, HVAC system 100 may additionally, or alternatively, include one or more valves for controlling the direction and/or rate of refrigerant flow within HVAC system 100. For example, in an embodiment of HVAC system 100, HVAC system 100 may be additionally provided with a reversing valve as well as additional piping sections to accommodate bi-directional refrigerant flow capability within HVAC system 100. Those of ordinary skill in the art will appreciate that corresponding variations to the refrigerant piping configuration may be provided to accommodate the particular component configuration of HVAC system 100 embodiment provided.

As shown in the embodiment of FIG. 1, HVAC system 100 may include evaporator 170. Evaporator 170 may provide for heat transfer between the refrigerant of HVAC system 100 and airflow passing over evaporator 170, as part of the vapor compression cycle. In an embodiment, evaporator 170 may be a heat exchanger coil assembly of any known type commonly used in HVAC systems, such as a fin-and-tube heat exchanger coil, a microchannel heat exchanger coil, and the like. The evaporator coil may comprise copper tubing with aluminum fins. Alternatively, evaporator coil may comprise all-aluminum tubing. Evaporator coil may comprise any suitable materials commonly used in HVAC systems.

Evaporator 170 may couple with, and receive refrigerant via refrigerant line 155. Refrigerant line 155 may comprise a low pressure pipe. According to the component configuration and refrigerant flow directions shown in FIG. 1, HVAC system 100 may be configured for operating in cooling mode. During cooling operation, refrigerant flowing through evaporator 170 may boil, changing from the 2-phase state to the gaseous state, as part of the vapor compression cycle. Gaseous refrigerant may be routed to compressor 120 via a common suction pipe to complete the refrigerant flow cycle within HVAC system 100 when operating in cooling mode.

HVAC system 100, as shown, may be a cooling-only unit or, alternatively, may be a heat pump unit operating in cooling mode. In alternative embodiments, HVAC system 100 may be configured to operate in heating mode as part of a heating only, or a heat pump unit, for example. In such embodiments, evaporator 170 may be configured to operate as a condenser as part of the vapor compression cycle, with the refrigerant flow directed in the opposite direction than that shown.

As shown in FIG. 1, in an embodiment, HVAC system 100 may include one or more sensors 180. Each sensor may sense and/or measure one or more parameter values of characteristics of HVAC system 100. The parameter value may indicate a condition of the refrigerant of HVAC system 100 and/or a condition of the outdoor ambient air within the outdoor unit 110. In the illustrated embodiment of FIG. 1, sensor 180 is a pressure sensor that may be configured to sense the refrigerant head pressure at a refrigerant line. Sensor 180 may measure head pressure at any suitable portion of the refrigerant line, such as at a first portion (e.g., between condenser 130 and outdoor expansion valve 140) and/or a second portion (e.g., between outdoor expansion valve 140 and indoor expansion valve 160). In some embodiments, sensor 180 may be configured to sense the ambient temperature, the refrigerant temperature, ambient air or refrigerant flow rates, and the like.

Sensor 180 may be a remote sensing device which may connect with controller 112 via a wired or wireless connection for transmitting sensed, or measured, data to controller 112. Sensor 180 may transmit analog or pneumatic signals either directly, or indirectly, to controller 112. In such an embodiment, the signals transmitted by sensor 180 may be converted to digital signals prior to use by controller 112. Alternatively, in an embodiment, sensor 180 may transmit digital signals to controller 112. In such an embodiment, the digital signals transmitted by the sensors 180 may be processed prior to use by controller 112 to convert the signals to a different voltage, to remove interference from the circuits, to amplify the signals, or other similar forms of digital signal processing. For each alternative described, herein, the signals of sensor 180 may be transmitted to controller 112 directly or indirectly, such as through one or more intermediary devices.

In an embodiment, sensor 180 may be disposed within the outdoor unit 110 and may be configured to sense the outdoor ambient air temperature within the outdoor unit 110. In such an embodiment, sensor 180 may be a thermistor. Alternatively, in such an embodiment, sensor 180 may be a thermocouple, a resistance temperature detection sensor, pyrometric sensor, an infrared thermographic sensor, or some other sensor type for sensing temperature values of outdoor ambient air. The sensor may transmit the sensed ambient temperature to controller 112 for use by controller 112 as input to one or more control methods. For example, controller 112 may use ambient temperature data received from sensor 180 as an input to a control method for setting the speed of motor 134 during the operation of HVAC system 100 to control the rate of heat transfer to, or from, refrigerant within the outdoor unit 110.

In alternative embodiments, sensor 180 may be disposed within HVAC system 100 at a position different from that shown in the particular embodiment of FIG. 1. For example, sensor 180 may be disposed within an indoor section of HVAC system 100 or may be coupled to a section of refrigerant piping within HVAC system 100. Further, in alternative embodiments, HVAC system 100 may be provided with additional sensing devices similar to sensor 180 for sensing parameter values indicating conditions of the refrigerant of HVAC system 100, ambient air, indoor return air, and the like. The sensing devices may be configured to sense temperature, pressure, flow rate, relative humidity, and other like parameter values. Additional sensing devices may be disposed within HVAC system 100 at any location. For example, sensor 180 may be disposed at evaporator 170, at outdoor expansion valve 140, at indoor expansion valve 160, within the conditioned space, and/or coupled to refrigerant piping. The additional sensing devices provided may connect to, and communicate with, controller 112 or, alternatively, may operate independently of controller 112, as described above. It will also be appreciated that some of the control methods described herein may require that HVAC system 100 be provided with one or more sensors 180 as shown or described, herein.

In alternative embodiments, HVAC system 100 may be provided with component configuration differing from that shown in the embodiment of FIG. 1. For example, HVAC system 100 may include: additional outdoor units 110; compressors 120, such as in a tandem compressor group; additional condensers 130; additional indoor expansion valves 160 and evaporators 170, such as in VRF systems; and the like. Further, in alternative embodiments, HVAC system 100 may be provided with a different component configuration than is shown in the embodiment of FIG. 1.

Referring now to FIG. 2, HVAC system 100 may include one or more controllers 200 that control operation of one, some, or all components within the system to meet a demand. In the illustrated embodiment of FIG. 2, controller 200 includes a memory 210, a processor 220, and an interface 230. In an embodiment, controller 200 may comprise, or be coupled to, a computer-readable medium with memory 210 for storing control logic or instructions for operating HVAC system 100 components. Controller memory 210 may be a volatile or non-volatile memory of any known type commonly used in HVAC systems. Controller 200 may store computer executable instructions within memory 210. The computer executable instructions may be included in computer code. Controller 200 may be implemented with hardware, software, firmware, or any combination thereof.

Controller 200 may, additionally, be implemented with processor 220 for executing stored instructions. Controller 200 may be responsive to or operable to execute instructions stored as part of software, hardware, integrated circuits, firmware, micro-code or the like. The functions, acts, methods or tasks performed by controller 200, as described herein, may be performed by processor 220 executing instructions stored in memory 210. The instructions are for implementing the processes, techniques, methods, or acts described herein. Controller processor 220 may be any known type of processor commonly used in HVAC systems. The processor may be a single device or a combination of devices, such as associated with a network or distributed processing. Controller 200 may operably couple to HVAC system 100 components via wired or wireless connections.

Controller 200 may receive data, which may comprise signals from one or more remote sensing devices. The data received by controller 200 may be received directly from one or more remote sensing devices, or, may be received indirectly through one or more intermediate devices such as a signal converter, a processor, an input/output interface (e.g. interface 230), an amplifier, a conditioning circuit, a connector, and the like. Controller 200 may operate HVAC system 100 components in response to received data from remote sensing devices. Additionally, controller 200 may operate HVAC system 100 components in response to user input, demands of the conditioned space, refrigerant and/or ambient air conditions, control logic, and the like.

Referring now to FIG. 3, HVAC system 300 illustrates an example embodiment of an HVAC system that includes multiple indoor units connected to one outdoor unit, such as in a VRF system. In the illustrated embodiment, HVAC system 300 includes an outdoor unit 310, a controller 312, a compressor 320, a discharge line 326, a condenser 330, fans 332 a-n, fan motors 334 a-n, an outdoor expansion valve 340, a hot gas bypass valve 350, a refrigerant line 355, indoor expansion valves 360 a-n, evaporators 370 a-n, and pressure sensor 380. As shown, outdoor unit 310, compressor 320, condenser 330, outdoor fans 332 a-n, fan motors 334 a-n, outdoor expansion valve 340, hot gas bypass valve 350, and pressure sensor 380 are located outside, whereas indoor expansion valves 360 a-n and evaporators 370 a-n are located inside. In other embodiments, one or more of the HVAC components shown outside may be located or partially located inside. Similarly, one or more of the HVAC components shown inside may be located or partially located outside. One or more components of HVAC system 300 may share attributes with like components of HVAC system 100.

In the illustrated embodiment of FIG. 3, each indoor evaporator has its own indoor expansion valve. For example, indoor expansion valve 360 a is coupled to evaporator 370 a, indoor expansion valve 360 b is coupled to evaporator 370 b, and so on. As shown, refrigerant line 355 supplies refrigerant to evaporators 370 a-n. Refrigerant line 355 may comprise one or more branches. The illustrated embodiment of HVAC system 300 includes reversing valve 324 as well as additional piping sections to accommodate bi-directional refrigerant flow capability within HVAC system 300.

As shown in FIG. 3, HVAC system 300 may include multiple outdoor fans 332 a-n. In an embodiment, each fan or fan bank may be coupled to a fan motor. For example, fan 332 a may be coupled to fan motor 334 a, fan 332 b may be coupled to fan motor 334 b, and so on. In some embodiments, the plurality of fans 332 a-n and/or the plurality of fan motors 334 a-n may have a single pre-determined minimum and/or maximum operating fan speed. Alternatively, or additionally, one or more of fans 332 a-n or fan motors 334 a-n may have a pre-determined minimum and/or maximum operating fan speed. For example, in an embodiment where a condenser has six fans, four of the six fans may be cycled off and the pre-determined minimum operating fan speed may be based on the fan speed of 2 operating fans.

Referring to FIG. 4, a flowchart of an example method 400 for controlling head pressure in an HVAC system during low ambient conditions is shown. The method 400 may be implemented in the order shown or, alternatively, may be implemented in an order different than that shown. In alternative embodiments, additional, fewer, or different steps of the method 400 may be provided in accordance with the alternative inputs, functions, and/or actions taken included in the discussion, below. Method 400 of FIG. 4 may be implemented by one or more processors (e.g., processor 220) to control head pressure of an HVAC system (e.g., HVAC system 100 or HVAC system 300) during low ambient conditions.

Method 400 starts at step 402. At step 402, method 400 is in normal cooling mode operation. In certain embodiments, during normal cooling mode operation, the outdoor expansion valve (e.g., outdoor expansion valve 140 or outdoor expansion valve 340) is fully open and the hot gas bypass valve (e.g., hot gas bypass valve 150 or hot gas bypass valve 350) is fully closed. Thus, refrigerant passes through the condenser (e.g., condenser 130 or 330), and the condenser lowers the temperature and pressure of the refrigerant. During normal cooling mode operation, the fan speed of outdoor fans can be adjusted to control the amount of cooling in order to maintain a proper pressure. As the fan speed decreases, less ambient air is circulated across the condenser, which results in less cooling and less temperature reduction. In general, the required fan speed tends to be relatively low in low ambient conditions, such as when the outdoor ambient air is freezing, because exposure to the low ambient conditions causes some heat loss.

At step 404, the processor determines whether a fan speed of one or more outdoor fans (e.g., fan 132 and/or fan 232) is less than or equal to a pre-determined minimum operating fan speed. For example, the one or more outdoor fans may be operating at a pre-determined minimum operating fan speed during low ambient conditions, such as when the outdoor temperature is at or between 0 degrees Fahrenheit and 23 degrees Fahrenheit. If the processor determines that the speed of the one or more outdoor fans is greater than the pre-determined minimum operating fan speed, the method stays in normal cooling mode operation. The method may repeat step 404 periodically, for example, according to a pre-determined time period or in response to a change in the fan speed. If at step 404 the processor determines that the speed of the one or more outdoor fans is less than or equal to the pre-determined minimum operating fan speed, the method moves to step 406.

At step 406 of method 400, as illustrated in FIG. 4, the processor receives a measurement of a first head pressure. As an example, the processor may receive a head pressure measurement of 180 psig at a refrigerant line (e.g., refrigerant line 155 or refrigerant line 355). The method then moves to step 408. At step 408, the processor determines whether the first head pressure is below a pre-determined minimum head pressure threshold. If the processor determines that the first head pressure is not below the pre-determined minimum head pressure threshold, the method may return to step 402. However, if the processor determines that the first head pressure is below the pre-determined minimum head pressure threshold, the method moves to step 410. For example, the processor may determine that the head pressure is below the pre-determined minimum head pressure threshold if the head pressure measurement is 180 psig and the pre-determined minimum head pressure threshold is 200 psig.

At step 410 of FIG. 400, the processor sends an instruction to at least partially open a hot gas bypass valve (e.g., hot gas bypass valve 150 and hot gas bypass valve 350). Thus, at least part of the refrigerant bypasses the condenser. The refrigerant that bypasses the condenser maintains a higher temperature and pressure than refrigerant that passes through the condenser. The refrigerant that bypasses the condenser and refrigerant that passes through the condenser can be combined in a refrigerant line (e.g., refrigerant line 155). In certain embodiments, the ratio of refrigerant that bypasses the condenser to refrigerant that does not bypass the condenser can be adjusted to maintain an appropriate pressure in the refrigerant line. The ratio can be adjusted by opening the hot gas bypass valve, closing the hot gas bypass valve, or adjusting the degree to which the hot gas bypass valve is open (e.g., in embodiments where the valve can be partially opened).

The method then moves to step 412, where the processor receives a measurement of a second head pressure. As shown in FIG. 400, the method then moves to step 414, where the processor determines whether the second head pressure exceeds a pre-determined maximum head pressure threshold. If the processor determines that the second head pressure exceeds the pre-determined maximum head pressure threshold, the method moves to step 416. At step 416, the processor sends an instruction to increase the speed of one or more outdoor fans. For example, if the head pressure measurement is 600 psig and the maximum head pressure threshold is 500 psig, the processor sends an instruction to increase the speed of the one or more outdoor fans. Increasing the outdoor fan speed causes more air to circulate across the condenser so that refrigerant through the condenser can be cooled, thereby lowering the pressure.

From step 416, method 400 of FIG. 4 moves to step 418, where the processor determines whether one or more outdoor fans is operating at a pre-determined maximum speed. If the processor determines that the one or more outdoor fans is operating at a pre-determined maximum speed, the method returns to step 402. In certain embodiments, the method resumes normal cooling mode operation at step 402. For example, the hot gas bypass valve may be closed, the outdoor expansion valve may be opened, and the temperature and pressure of the refrigerant may be adjusted by controlling the outdoor fan speed. If the processor determines that the one or more outdoor fans is not operating at a pre-determined maximum speed, the method moves to step 412, where the processor receives a measurement of a third head pressure at the refrigerant line.

Alternatively, if at step 414 the processor determines that the second head pressure does not exceed the pre-determined maximum head pressure threshold, the method moves to step 420. At step 420, the processor determines whether the second head pressure is below the pre-determined minimum head pressure threshold. If the processor determines that the second head pressure is below the pre-determined minimum head pressure threshold, method 400 moves to step 422. At step 422, the processor sends an instruction to at least partially close the outdoor expansion valve. As an example, if the head pressure measurement is 190 psig and the minimum head pressure threshold is 200 psig, the processor may send an instruction to fully close the outdoor expansion valve. In certain embodiments, closing the outdoor expansion valve may cause the amount of refrigerant that passes through the condenser to decrease such that the amount of refrigerant that passes through the hot gas bypass valve increases. Thus, higher temperature, higher pressure refrigerant may be provided to the refrigerant line.

Alternatively, if at step 420 the processor determines that the second head pressure is not below the pre-determined minimum head pressure threshold, the method moves to step 412, where the processor receives a measurement of a third head pressure. The third measurement may be compared to a maximum head pressure threshold at step 414. The method may repeat the steps as necessary and may adjust the outdoor fan speed, the hot gas bypass valve, and the outdoor expansion valve to maintain a proper pressure in the refrigerant line.

In the preceding discussion, numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it will be understood that the present disclosure may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present disclosure in unnecessary detail. Additionally, for the most part, details concerning well-known features and elements have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present disclosure, and are considered to be within the understanding of persons of ordinary skill in the relevant art.

Having thus described the present disclosure by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present disclosure may be employed without a corresponding use of other features. Many such variations and modifications may be recognized based upon a review of the foregoing description of preferred embodiments. 

1. An HVAC system for controlling head pressure during low ambient conditions, comprising: a condenser operable to receive refrigerant from a discharge line, condense the refrigerant, and discharge the refrigerant to a first portion of a refrigerant line; an outdoor expansion valve between the first portion of the refrigerant line and a second portion of the refrigerant line; an indoor expansion valve between the second portion of the refrigerant line and a third portion of the refrigerant line; a hot gas bypass valve coupled to the discharge line and the second portion of the refrigerant line such that, when open, refrigerant through the hot gas bypass valve bypasses the condenser and the outdoor expansion valve; one or more outdoor fans operable to cool the condenser; one or more outdoor fan motors operable to control the fan speed of the one or more outdoor fans, wherein the one or more outdoor fan motors has a pre-determined minimum operating fan speed; and one or more controllers operable to: receive a measurement of a first head pressure at the refrigerant line; determine whether the first head pressure is below a pre-determined minimum head pressure threshold; and send an instruction to at least partially open the hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold; and a speed of the one or more outdoor fan motors is less than or equal to the pre-determined minimum operating fan speed.
 2. The system of claim 1, wherein the controller is further operable to: receive a measurement of a second head pressure at the refrigerant line after at least partially opening the hot gas bypass valve; determine whether the second head pressure exceeds a pre-determined maximum head pressure threshold; and send an instruction to increase the speed of the one or more outdoor fan motors in response to a determination that the second head pressure exceeds the pre-determined maximum head pressure threshold.
 3. The system of claim 1, wherein the controller is further operable to: receive a measurement of a second head pressure at the refrigerant line after opening the hot gas bypass valve; determine whether the second head pressure is below the pre-determined minimum head pressure threshold; and send an instruction to at least partially close an outdoor expansion valve in response to a determination that the second head pressure is below the pre-determined minimum head pressure threshold.
 4. The system of claim 3, wherein the controller is further operable to: receive a measurement of a third head pressure at the refrigerant line after at least partially closing the outdoor expansion valve; determine whether the third head pressure exceeds the pre-determined maximum head pressure threshold; send an instruction to increase the speed of the one or more outdoor fan motors in response to a determination that the third head pressure exceeds the pre-determined maximum head pressure threshold.
 5. The system of claim 4, wherein the controller is further operable to determine that the increased speed of the one or more outdoor fan motors equals a pre-determined maximum operating speed of the one or more outdoor fan motors and, in response, send at least one of: an instruction to at least partially open the at least partially closed outdoor expansion valve; and an instruction to at least partially close the at least partially open hot gas bypass valve.
 6. The system of claim 5, wherein at least partially opening the outdoor expansion valve comprises fully opening the outdoor expansion valve.
 7. The system of claim 1, wherein at least partially opening the hot gas bypass valve comprises fully opening the hot gas bypass valve.
 8. The system of claim 1, wherein the system's capacity ranges from 6 to 36 tons, the minimum head pressure threshold is a value between 200 and 300 pounds per square inch gauge (“psig”), and the maximum head pressure threshold is a value between 400 and 500 psig.
 9. The system of claim 1, wherein the low ambient conditions comprise outdoor temperatures ranging from 0 degrees Fahrenheit to 23 degrees Fahrenheit.
 10. The system of claim 1, wherein the HVAC system is a variable refrigerant flow system and the third portion of the refrigerant line is coupled to two or more indoor expansion valves, each indoor expansion valve associated with a distinct evaporator of a plurality of evaporators.
 11. A method for controlling head pressure in an HVAC system during low ambient conditions, comprising: receiving, by a processor, a measurement of a first head pressure at a refrigerant line; determining, by the processor, whether the first head pressure is below a pre-determined minimum head pressure threshold; and sending, by the processor, an instruction to at least partially open a hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold; and a speed of one or more outdoor fans is less than or equal to a pre-determined minimum operating speed of the one or more outdoor fans.
 12. The method of claim 11, further comprising: receiving, by the processor, a measurement of a second head pressure at the refrigerant line after at least partially opening the hot gas bypass valve; determining, by the processor, whether the second head pressure exceeds a pre-determined maximum head pressure threshold; and sending, by the processor, an instruction to increase the speed of the one or more outdoor fans in response to a determination that the second head pressure exceeds the pre-determined maximum head pressure threshold.
 13. The method of claim 11, further comprising: receiving, by the processor, a measurement of a second head pressure at the refrigerant line after opening the hot gas bypass valve; determining, by the processor, whether the second head pressure is below the pre-determined minimum head pressure threshold; and sending, by the processor, an instruction to at least partially close an outdoor expansion valve in response to a determination that the second head pressure is below the pre-determined minimum head pressure threshold.
 14. The method of claim 13, further comprising: receiving, by the processor, a measurement of a third head pressure at the refrigerant line after at least partially closing the outdoor expansion valve; determining, by the processor, whether the third head pressure exceeds the pre-determined maximum head pressure threshold; sending, by the processor, an instruction to increase the speed of the one or more outdoor fans in response to a determination that the third head pressure exceeds the pre-determined maximum head pressure threshold.
 15. The method of claim 14, further comprising determining that the increased speed of the one or more outdoor fan motors equals a pre-determined maximum operating speed of the one or more outdoor fan motors and, in response, sending at least one of: an instruction to at least partially open the at least partially closed outdoor expansion valve; and an instruction to at least partially close the at least partially open hot gas bypass valve.
 16. The method of claim 15, wherein at least partially opening the outdoor expansion valve comprises fully opening the outdoor expansion valve.
 17. The method of claim 11, wherein at least partially opening the hot gas bypass valve comprises fully opening the hot gas bypass valve.
 18. The method of claim 11, wherein the system's capacity ranges from 6 to 36 tons, the minimum head pressure threshold is a value between 200 and 300 pounds per square inch gauge (“psig”), and the maximum head pressure threshold is a value between 400 and 500 psig.
 19. The method of claim 11, wherein the low ambient conditions comprise outdoor temperatures ranging from 0 degrees Fahrenheit to 23 degrees Fahrenheit.
 20. A non-transitory computer readable medium comprising logic that, when executed by a processor, is operable to: receive a measurement of a first head pressure at a refrigerant line; determine whether the first head pressure is below a pre-determined minimum head pressure threshold; and send an instruction to at least partially open a hot gas bypass valve in response to a determination that: the first head pressure is below the pre-determined minimum head pressure threshold; and a speed of one or more outdoor fans is less than or equal to a pre-determined minimum operating speed of the one or more outdoor fans. 